Why would something not work in a “modern context”? What do you mean by that? Unless a technique is designed to rely on an obsolete implement, like a special grip on a piece of clothing that was common 200 years ago but nobody wears anymore, then why would a technique not work, if you have developed yours skill with it? That is what it always comes down to: you need to develop the skill. It isn’t that things inherently work or do not work. The ability to use something or not use something, lies with the individual.
Likewise, why would something not work against MMA? What do you mean by that? In the context of an MMA competition? Simply against someone who has trained to be an MMA competitor? Once again, you need to develop the skill to use it, but if you do so then of course you can use it successfully, if your skill is greater than your enemy. Simply being an MMA person doesn’t put one on a pedestal and suddenly eliminate the possibility of using certain techniques against him. MMA folks get hurt just like anyone else. Some things may be more risky to try in the context of an MMA competition, but that is an different issue and doesn’t mean that it simply does not work in a “modern context” or against MMA.
There are plenty of reasons things might not work in a modern context, and this is linked to the MMA practitioner idea; I'm not putting them on any pedestal.
Techniques- some techniques which are common now, weren't when certain styles were developed. As an example, roundhouse kicks were not a thing in northern China hundreds of years ago, so the kick defence approaches are very different.
Clothing- Certain techniques were indeed developed with certain clothing, armour or even hairstyles in mind. This could be your opponent wearing it or you. There are techniques which assume you are wearing heavy gauntlets to protect your hand, others that pull on helmet spikes etc.
Weapons- Certain techniques assume defence against a weapon e.g. a spear. Some systems built their hand techniques based on weapons fighting so that both could be taught at once.
Group Fighting- Some styles originate in the assumption that when fighting it would generally be in a group. This necessitates a slightly different approach than one on one
Sports- Not wanting to get into the whole sports applicability or groin shot arguments, but sports style fighting assumes 2 willing participants within a limited area and certain rules. Of course sports fighting is applicable outside of this context, but some techniques in traditional martial arts are based on the assumption that you are fighting outside, don't have space limits, are able to strike soft targets to gain an advantage, or aren't a willing participant. This means different footwork, different scales of movement etc.
So take all of that together and you have techniques which may not be particularly useful in some friendly indoor sparring against a guy that strikes like a kickboxer or muay thai fighter. Of course some of them might still be useful and we'd have to try them to find out, but I can tell you right now that I'm not going to use look back and enjoy the moon into a duck step whilst sparring, because I'd run out of space and I don't want to hit my training partner in the groin.